BACKGROUND OF THE INVENTION
This invention pertains to an electrically insulating non-woven fabric comprising an aromatic polyamide fiber (aramid fiber) non-woven fabric, a prepreg and a laminate having the aforementioned electrically insulating non-woven fabric as base material and including a printed wiring board and a multi-layer printed wiring board as a concept on which leadless chip parts such as resistors, ICs and so on are surface-mounted.
Of late, in case that electronic parts such as resistors, ICs and so on are mounted on a printed wiring board which is assembled in an electronic device or instrument, the electronic parts have generally tended to be chipped and mounted on the printed wiring board by a surface-mount system. The surface-mount system is a preferable one because the electronic parts are compacted and lightened and have a high density. As the printed wiring board has the high density, it tends to be generally worked by an IVH (which stands for an inner via hole not extending through an entire multilayer insulation, but partially extending through the insulation) process in which a laser light is irradiated on the printed wiring board in order to connect electrically conducting wires to each other through an IVH provided in an insulating layer disposed between the electrically conducting wires among the multilayer insulation. Thus, the printed wiring board is required to be easily worked by the laser process.
In case that the leadless chip parts are surface-mounted on the printed wiring board, the printed wiring board should have a coefficient of thermal expansion matched to that (2xcx9c7 ppm/xc2x0 C.) of the leadless chip parts as much as possible.
The printed wiring board is desired to have a smaller ratio of size variation due to thermal shrinkage for improving a reliance on connection of the electrically conducting wires between which the insulating layer is provided. Particularly, the multi-layer printed wiring board makes much of the smaller ratio of size variation.
In view of this, there have been developed various laminates having base material formed of non-woven fabrics of aramid fibers having a negative coefficient of thermal expansion. These non-woven fabrics are as follows;
(1) a mixed non-woven fabric of para-aramid fiber chops such as poly-p-phenylene-3,4xe2x80x2-diphenylether-terephthalamide fiber chops and thermoplastic resin fiber chops having a softening temperature of 220xc2x0 C. or higher and these fibers being bonded by a resin binder while the thermoplastic resin fiber chops having the softening temperature of 220xc2x0 C. or higher are thermally adhered to the para-aramid fiber chops which is disclosed in JP 10-138381.
(2) a mixed non-woven fabric of para-aramid fiber chops such as poly-p-phenylene-terephthalamide fiber chops, para-aramid fiber pulps and meta-aramid fibrids and the para-aramid fiber pulps and the meta-aramid fibrids being intertwined with the para-aramid fiber chops which is disclosed in JP61-160500.
Prepregs are produced by impregnating the non-woven fabrics with a thermosetting resin and drying them. The thus produced prepregs are heated and pressed to form a laminate. Generally, a metal foil or foils to be worked for forming printed wirings are provided integrally with the prepregs when they are formed under heat and pressure so as to form a metal foil clad laminate.
Poly-p-phenylene-3,4xe2x80x2-diphenylether-terephthalamide fibers which are one kind of the para-aramid fibers should be drawn for improving a strength of fibers when spinning them, but the drawn fibers tend to be shrunk due to heat applied thereto. Thus, the printed wiring board having the base insulation material of the non-woven fabrics including poly-p-phenylene-3,4xe2x80x2-diphenylether-terephthalamide fibers as a principal component tends to have a higher size variation or be thermally shrunk in high degree when solder reflows. Such a printed wiring board should be more improved in a reliance on connection of printed wirings through the insulating layer as well as connection of the surface-mounted parts to each other.
Poly-p-phenylene-terephthalamide fibers which are another kind of the para-aramid fibers have a very high crystallinity because they are crystally spun and therefore have a high bond between molecules. However, the printed wiring board having the base insulation material of the non-woven fabrics including poly-p-phenylene-terephthalamide fibers as a principal component tends to have a lower hole making operation by a laser irradiation process. As a matter of course, prepregs produced by impregnating the non-woven fabrics with a thermosetting resin and drying them also have a lower hole making operation by a laser irradiation process.
More particularly, when a prepreg formed of the non-woven fabric including poly-p-phenylene-terephthalamide fibers and a laminate formed of the thus produced prepregs are worked by a hole-making operation by the laser process to form holes, they have a large dispersion or scattering in the resultant hole diameter. This is caused by the low degree in which aramid fibers are discomposed and scattered by the irradiation of the laser. These fibers have little shrinkage due to heat because they are spun in a crystalline manner, but not in a drawn manner.
Accordingly, it is a principal object of the invention to provide an electrically insulating non-woven fabric having para-aramid fibers as a principal component suitable for forming a prepreg or a laminate including a printed wiring board adapted to have a good hole-making operation by a laser process while it has little dispersion in hole diameter.
It is another object of the invention to provide an electrically insulating non-woven fabric having para-aramid fibers as a principal component suitable for forming a laminate including a printed wiring board adapted to provide no dispersion in size variation due to heat applied to the laminate.
It is further object of the invention to provide a prepreg comprising as a base material a non-woven fabric having para-aramid fibers as a principal component adapted to have a good hole-making operation by a laser process while it has little dispersion in hole diameter.
It is further object of the invention to provide a laminate formed of prepregs comprising as a base material a non-woven fabric having para-aramid fibers as a principal component adapted to have a good hole-making operation by a laser process while it has little dispersion in hole diameter and adapted to have little dispersion in size variation due to heat.
In accordance with a first aspect of the invention, there is provided an electrically insulating non-woven fabric comprising para-aramid fiber chops bonded with each other by a binder, the non-woven fabric characterized by the para-aramid fiber chops having a mixture of poly-p-phenylene-diphenylether-terephthalamide fibers and poly-p-phenylene-terephthalamide fibers with a blend ratio by weight of poly-p-phenylene-diphenylether-terephthalamide fibers relative to poly-p-phenylene-terephthalamide fibers ranging from 10/90 to 90/10.
The poly-p-phenylene-diphenylether-terephthalamide fibers may be specifically poly-p-phenylene-3,4xe2x80x2-diphenylether-terephthalamide fibers.
The blend ratio by weight of poly-p-phenylene-diphenylether-terephthalamide fibers relative to poly-p-phenylene-terephthalamide fibers may preferably range from 30/70 to 70/30.
The binder serving to bond the fibers to each other may be selectively a thermosetting resin binder or a thermoplastic resin having a softening temperature of 220xc2x0 C. or higher. Otherwise, both of them may be used as the binder. The thermosetting resin binder bonds the fibers to each other by being attached to the fibers at their intersections. The thermoplastic resin having the softening temperature of 220xc2x0 C. or higher bonds the fibers to each other by being thermally adhered to and/or being intertwined with the para-aramid fiber chops.
The thermoplastic resin having the softening temperature of 220xc2x0 C. or higher may be in the form of at least one selected from chops, fibrids and pulps.
The chops are formed by cutting straight fibers into ones having predetermined size suitable for paper making. The xe2x80x9cfibridsxe2x80x9d are small non-granular, non-rigid fibrous or film like particles, as defined in U.S. Pat. Nos. 4,698,267 and 4,729,921. The fibrids may be formed by beating a film-like resin or may be NOMEX fibrids commercially available from Du Pont or CONEX fibrids commercially available from Teijin, Japan. The pulps are formed by beating fibers.
The chops can be intertwined with each other by being deformed due to thermal adhesion or thermal softening so that the fibers can be bonded to each other. The fibrids and the pulps themselves functions to be intertwined with each other. Thus, the fibers can be bonded to each other by paper making them together with para-aramid fiber chops. They can be more strongly intertwined with each other by deformation due to thermal adhesion or thermal softening by appropriately applying heat thereto. Para-aramid fiber pulps may be blended as an auxiliary component of the binder.
With the poly-p-phenylene-diphenylether-terephthalamide fibers and the poly-p-phenylene-terephthalamide fibers in combination used as the para-aramid fiber chops, the poly-p-phenylene-terephthalamide fibers serve to restrain the size variation due to heat applied thereto. Since all the para-aramid fiber chops are not just the poly-p-phenylene-terephthalamide fibers, the products are allowed to be worked by the laser process.
In general, the non-woven fabric tends to have high or low fiber density on its spots. More particularly, it has the areas where more fibers occupy and the areas where less fibers occupy. A diameter of hole formed by the laser process varies on the fiber density thereof. As there increases the blend ratio of poly-p-phenylene-terephthalamide fibers which have bad working by the laser process, the fiber density of the non-woven fabric makes a much effect of hole-making by the laser process, which causes a dispersion in the hole diameter to vary on the positions of the non-woven fabric.
With the blend ratio of poly-p-phenylene-terephthalamide fibers being 90 or less while that of poly-p-phenylene-diphenylether-terephthalamide fibers is 10 or more, the dispersion in the hole diameter gets smaller.
With the blend ratio of poly-p-phenylene-diphenylether-terephthalamide fibers being 90 or less while that of poly-p-phenylene-terephthalamide fibers is 10 or more, a dispersion in the size variation due to heat gets smaller and the reliance in the IVH and the part surface-mount is improved.
Thus, it will be noted that both of the characteristics with respect to the dispersion in the hole diameter in the hole-making operation by the laser process and the dispersion in the size variation due to heat can be improved by the blend ratio by weight of the two para-aramid fibers ranging from 10/90 to 90/10 so that no special care is taken of the laser process and the solder reflowing. The blend ratio by weight of the former relative to latter ranging from 30/70 to 70/30 can make less dispersions in the hole diameter by the laser process and in the size variation due to heat and also less size variation itself due the heat.
The prepregs according to the invention are formed by impregnating the electrically insulating non-woven fabrics with a thermosetting resin and drying them and the electrically non-woven fabrics include at least one constructed in accordance with the invention as aforementioned.
The laminate according to the invention is formed by forming a layer or layers of the prepreg under heat and pressure and the layer or layers of the prepreg include at least one constructed in accordance with the invention as aforementioned.